Hi, as you said, there is no slot for a memory card on the iPad, and there is no app that would allow the iPad to apply the data recovery tool on such a memory card.
You will need a computer in order to do what you are trying to accomplish.
(1)For Mac, it's highly recommended to use iBoysoft Data Recovery for Mac, which can recover deleted files of over 1000 types on an SD card, external hard drive, or HDD…
(2)If you have a PC, try iBoysoft Data Recovery, a professional data rescue tool for SD card recovery.
Try it on your computer and hope that you can find all the files in the end.
Also read: Best SD Card Recovery Apps and Guidelines
How to Recover Deleted Files from SD Card on Windows & Mac?